Speedometer calibration

[Elisabeth]

I suspect that my speedometer/odometer s calibrated for a 26" wheel. How would I adjust it to my 20" fat tire rim

[inoxa]

I suspect that my speedometer/odometer s calibrated for a 26" wheel. How would I adjust it to my 20" fat tire rim

You need to measure the tire diameter not the rim.

[handlebar]

Adjustment would depend on what display you have.

I found the diameter of my "20 inch" tire by measuring from the center of the axle to the floor, with the tire touching the ground. Doubling it showed a diameter of 23 inches.

[John Rose]

I suspect that my speedometer/odometer s calibrated for a 26" wheel. How would I adjust it to my 20" fat tire rim

For this style of display and separate PAS control with three buttons, press MODE to turn it on, press and hold UP and DOWN buttons simultaneously to get to the settings menu. The first setting is for tire diameter. Press UP to increase, or DOWN to decrease. Press MODE [and hold?] to save and exit.

[Naranja_CT5+]

Even if you are using the proper diameter tire how would you even know you are getting the correct distance?
I remember a long time ago that there?s a road marker where it tells you 1km, 2km, etc for this exact purposes. But I haven?t seen these markers anywhere anymore.

While riding my regular bicycle I use an app called Speedometer 55. It?s great for an old fart like me because it has big coloured numbers. And it works on my old iPhones so I don?t have to worry of it getting stolen when I forget to take it out of the bicycle mount.

UPDATE: I was wrong. I have forgotten that walking/cycling paths on some parks DO have distance markers.

[handlebar]

Even if you are using the proper diameter tire how would you even know you are getting the correct distance?

I used Apple's Map app to measure a local circuit of streets. It's 2.0 miles, but i found that a I might have to roll blocks further for the odometer to accumulate 2 miles. What's more, some days it was more accurate than others.

Decades ago, I measured the local walking track with a tape. It's exactly 3/10 of a mile. A sign prohibits bicycles, but the track is pretty much abandoned and I'm a scofflaw. On my Radrunner, If I dropped a marker when the trip meter turned up a tenth of a mile, it would turn up 3/10s higher when I reached that marker again. My Radmission was just as accurate.

That was only true if I kept my speed down to about 6 mph, which was hard to do.  If I sped up a little, maybe 9 mph, I might have to go 30% more than a lap before the odometer added 3/10. If I want faster, say 12 mph, the odometer might register 3/10 when I'd been only 60% of a lap.

The neglected track is rough. On both bikes, I guess the magnetic wheel sensor in the motor gets erratic on bumps.

[John Rose]

I ended up buying the LCD Display upgrade for my RadExpand 5. I forget what wheel diameter it was set up for out of the box, but it was wrong.

I had changed it to the nominal 20", even though a 20" is what it measures about halfway between the rim and the tread. The axle to floor radius with nobody sitting on it is close to 11", which comes to approximately 22" diameter.

Today I found a straight level stretch of road, and when the GPS-based speedometer app was reporting 30 kph, the bike's display was showing between 27.2 to 28 kph. It kept changing, so it was hard to get a steady reading. The trip distance on the app was 1.02 km, versus 0.9 km on the bike display.

I stopped and changed the setting in the menu, from 20" to 22". Not surprisingly, the speedometer then indicated 36.7(ish) when the GPS speedometer app indicated 36 kph. I didn't want to reset the trip distance at that time, because reasons.
Next time, I'll compare the trip distances over, say, 10 km and see how close they are to each other.

At any rate, it appears that the most accurate speedometer & odometer setting is to have it agree with the measured axle height x 2.

This would make far more sense if you could dial in an arbitrary number for the diameter, say, to the nearest centimetre. As it is, there are only a limited number of diameters to choose from in the menu.

[John Rose]

Update...
Today I jumped on the bike to go to the hardware store to buy some fresh bird seed (as one does).
With nominally 4"x20" tires, at 20 psi, and a Sekrit Menu setting of 22" diameter, I got these results

• To the store: Bike odometer: 3.8 km / GPS app 3.78 (probably just a rounding error)
• Return trip: (a slightly longer route) Bike odometer 4.1 km / GPS app 4.1

I reset them before returning home because I was walking around in the store with my phone, which would introduce error in a comparison.

So, I'm sticking with the 22" setting.